This invention relates to a sound field control unit and a sound field control device which, on the basis of existing acoustic conditions in a room, supports a sound field by reinforcing acoustic effects such as sound volume, reverberation and acoustic spaceousness by electrical acoustic means and, more particularly, to a sound field control unit and a sound field control device of this type facilitating introduction of a sound field support system.
There is a prior art acoustic feedback system for electro-acoustically realizing a control such as prolongation of reverberation. The principle of this system is shown in FIG. 2. In this system, a loudspeaker 12 and a microphone 14 are provided with a proper distance therebetween in a room 10 and sound collected by the microphone 14 is supplied to an FIR (finite impulse response) filter 18 through a head amplifier 16 to produce a reverberation signal (mainly an initial reflected sound signal). This reverberation signal is supplied to the loudspeaker 12 through an amplifier 20 and reverberation sound sounded from the loudspeaker 12 is collected by the microphone 14. It is intended to realize, by repeating this process, feeling of increase in sound volume (i.e., increase in sound pressure level), increase in reverberation and increase in spaceousness of sound (i.e., increase in side reflected sound energy). According to this system, impression of a sound field as if the listener was listening to a musical performance in a large concert hall is created despite the fact that he is actually in a small room.
A prior art sound field control device using this acoustic feedback system is shown in FIGS. 3 and 4. FIG. 3 shows an arrangement of a microphone and a loudspeaker. In a room 22 such as a music chamber, four microphones 24 to 27 are disposed on the ceiling and four loudspeakers 30 to 33 are provided on the wall. A main body 36 of the device is constructed as an independent unit and is provided in the room 22.
FIG. 4 shows a circuit of this device. Signals collected by the microphones 24 to 27 are supplied through head amplifiers 38 to 41 to an equalizer 42 in which the signals are adjusted in frequency characteristics for preventing occurrence of howling. The signals are then supplied to an FIR filter 44 to produce initial reflected sound signals. The initial reflected sound signals are amplified by amplifiers 46 to 49 and sounded from the loudspeakers 30 to 33. A ROM 52 stores various initial reflected sound parameters of various sound field patterns. By selecting a sound field pattern by an outside infrared remote control transmitter-receiver 54, a corresponding sound field pattern selection signal 56 is transmitted and received by a receiving window 58 provided in the main body 36 of the device. A CPU 60 receives this signal and thereupon reads out an initial reflected sound parameter of the corresponding sound field and sets this parameter in the FIR filter 44 to establish a sound field space.
Adjustment of the frequency characteristics by the equalizer 42 is performed by either of the following methods:
(a) The equalizer 42 is constructed of an FIR filter. Characteristics of the room are previously measured and the characteristics of the FIR filter are preset to inverse characteristics of the characteristics of the room. Signals collected by the microphones 24 to 27 are subjected to convolution operation by the FIR filter 43 to flatten their frequency characteristics. PA1 (b) The equalizer 42 is constructed of a notch filter. When howling occurs during use of the device, the notch filter is applied at this time to the frequency region in which the howling has occurred to prevent howling. PA1 (c) The equalizer 42 is constructed of a graphic equalizer. An operating person adjusts the graphic equalizer manually to flatten the frequency characteristics while he measures the characteristics of the room.
In the prior art sound field control device employing the acoustic feedback system, location of microphones and loudspeakers is generally rearranged for securing safety against occurrence of howling when space (room) where they are placed has changed. In that case, locations of the microphones and the loudspeakers are changed and distance between the microphones and the loudspeakers is also changed each time space is changed. Adjustment after installation is conducted in such a manner that adjustments of open loop gain and transmission frequency characteristics are carried out for each combination of the microphones and loudspeakers by using special measuring instruments and then presence or absence of howling and coloration of tone quality are checked. Thus, installation and adjustment of the device require a long time and this becomes an obstacle to introducing the sound field support system.
Among the above described methods for adjusting frequency characteristics of the equalizer 42, the method using the FIR filter requires a high-class computer for dealing with a tremendous amount of calculation. The method using the notch filter cannot completely prevent occurrence of howling since the notch filter copes with howling after it has actually taken place. Moreover, in a case where many peaks exist over a broad frequency region in the frequency characteristics, a plurality of notch filters are required. The method using the graphic equalizer is very troublesome because adjustments are made manually.
It is, therefore, an object of the present invention to provide a sound field control unit and a sound field control device which can overcome the above described problems of the prior art device and facilitate introduction of the sound field support system.